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Comparison of corneal astigmatism measured with 3 devices. Mariko Shirayama, M.D, Li Wang, M.D, PhD, Mitchell P. Weikert, M.D, Douglas D. Koch, M.D. Cullen Eye Institute, Baylor College of Medicine, Houston, TX. Financial Interest Disclosure: Research support--Li Wang; - PowerPoint PPT Presentation
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Comparison of corneal astigmatism measured with 3 devices
Mariko Shirayama, M.D, Li Wang, M.D, PhD, Mitchell P. Weikert, M.D, Douglas D. Koch, M.D.
Cullen Eye Institute, Baylor College of Medicine, Houston, TX.Financial Interest Disclosure: Research support--Li Wang;
travel expenses—Mariko Shirayama
Calculating corneal astigmatism
Corneal astigmatism has been traditionally calculated from anterior corneal curvature using 1.3375 as index of refraction
New Scheimpflug images allow calculation of anterior and posterior corneal astigmatism
Purpose
To assess the repeatability and comparability of corneal astigmatism obtained from the
IOLMasterHumphrey Atlas corneal topographerGalilei Dual Scheimpflug Analyzer
To evaluate the effect of posterior corneal astigmatism on overall corneal astigmatism
Inclusion criteria: No prior intraocular and/or corneal
surgery No trauma, ocular or corneal diseases No contact lens wear
21 eyes of 21 patients included Gender: 6 males and 14 females Age: 34±11.4 yrs (range 18 to 59 yrs)
Subjects
Methods Prospective study
Corneas measured with: 4 techniques using 3 devices Single examiner 3 sets of corneal measurements each Subject repositioned between measurements
Humphrey Atlas Corneal Topographer
The IOLMaster
2) CAAtlas: Difference between steep and flat meridians of simulated keratometry readings from the Atlas
1) CAIOLMaster: Difference between steep and flat meridians of automated keratometry from the IOLMaster
Corneal astigmatism measurements
3) CAGalilei Sim: Difference between steep and flat meridians of simulated keratometry readings from the Galilei over the 1.0- 4.0mm diameter central zone
4) CAGalilei TCP: Difference between the steep and flat meridians of total corneal
power* over the 1.0-4.0-mm diameter central zone
*Total corneal power is calculated by ray-tracing through the anterior and posterior corneal surfaces using Snell’s law.
Galilei Dual Scheimpflug Analyzer
2 categories for astigmatism measurements
Astigmatism estimated from anterior corneal power using 1.3375 CAIOLMaster CAAtlas CAGalilei Sim
Astigmatism estimated from total corneal power
CAGalilei TCP
Data analysisRepeatability
Coefficient of variation (CV) Standard deviation (SD) Intraclass correlation coefficient (ICC) Vector analysis of astigmatism
measurements for each device
Data analysisComparability Comparison of mean corneal astigmatism in
magnitude between devices Paired t-test with Bonferroni correction
Agreement in measurements between devices Bland and Altman method
95% limits of agreement (95% LoA) calculated as mean difference ±
1.96 standard deviation (SD)
Interdevice correlation Pearson correlation coefficient
Data analysisEvaluation of effects of posterior corneal astigmatism on total corneal astigmatism Differences in meridian of anterior and posterior corneal astigmatism from the Galilei Ratio in curvatures of anterior and posterior corneal astigmatism Vector difference between CATCP and CASimK
Comparison of astigmatism estimated from Anterior and Total corneas
Results: Repeatability of 3 measurementsMethods CAIOLMaster CAAtlas
CAGalilei
Sim
CAGalilei
TCP
CV 21% 14% 28% 26%
SD 0.14D 0.08D 0.14D 0.13D
ICC 0.931 0.983 0.922 0.918
All techniques provided high repeatability, especially the Atlas
Vector differences between repeated measurements (% of eyes)
Difference between measurements ≤0.25D ≤0.50D ≤0.75D ≤1.00D
CAIOLMaster 71 93 95 100
CAAtlas 76 100 100 100
CAGalilei Sim 45 76 93 98
CAGalilei TCP 36 76 90 98
42 values for each device (21 for measurement 1-2 and 21 for measurement 1-3)
Atlas tended to provide smallest vector differences between repeated measurements
CAIOLMaster CAAtlas CAGalilei Sim CAGalilei TCP
Double-angle plots
Each ring=0.5DOuter ring=2.0D
Mean CA ± SD @ degree
0.46 ± 0.44@87
0.43 ± 0.40@88
0.37 ± 0.38@94
0.17 ± 0.38@77
Double-angle plots for the average astigmatism from each device
15
3045
60
135150
165
0 0
15
3045
60
75
90
105
120135
150
165
75
90
105
120
0
15
3045
60
75
90
105
120135
150
165
0
15
3045
60
75
90
105
120135
150
165
CAGalilei TCP tended to indicate lower astigmatism than other techniques
Mean corneal cylinder measured by each device
There were no significant differences between techniques although the IOLMaster tended to provide larger cylinder
Mean (D) SD (D) Range (D)
CAIOLMaster 0.74 0.41 0.11 - 1.49CAAtlas 0.70 0.39 0.20 - 1.48CAGalilei Sim 0.65 0.33 0.11 - 1.29CAGalilei TCP 0.59 0.31 0.12 - 1.12
Results: ComparabilityAgreement in corneal astigmatism measurements between devices
The ranges of the 95% LoA between CAIOLMaster and CAGalilei TCP, and CAAtlas and CAGalilei TCP were wider than other pairs
Diffe
renc
e in
cor
neal
ast
igm
atism
(D
) Mean corneal astigmatism (D) of the CAAtlas and CAIOLMaster
LoA=0.72
Mean corneal astigmatism (D) of the CAGalilei SimK and CAIOLMaster Di
ffere
nce
in c
orne
al a
stig
mat
ism (D
)
LoA=0.71 LoA=0.88
Mean corneal astigmatism (D) of the CAGalilei SimK and CAAtlas
Diffe
renc
e in
cor
neal
ast
igm
atism
(D
)
LoA=1.23
Mean corneal astigmatism (D) of the CAIOLMaster and CAGalilei TCP
Diffe
renc
e in
cor
neal
ast
igm
atism
(D
)
LoA=1.36
Diffe
renc
e in
cor
neal
ast
igm
atism
(D)
Mean corneal astigmatism (D) of the CAAtlas and CAGalilei TCP
LoA=0.74
Mean corneal astigmatism (D) of the CAGalilei SimK and CAGalilei TCP
Diffe
renc
e in
cor
neal
ast
igm
atism
(D)
Results: Mean vector difference between CAGalilei SimK and CAGalilei TCP
Astigmatism estimated from anterior vs. total cornea
CAGalilei SimK - CAGalilei TCP
Double-angle plotsEach ring=0.5DOuter ring=2.0D
0.21 ± 0.11D @ 860
15
3045
60
75
90
105
120135
150
165
Mean vector difference between anterior and total corneal astigmatismwas 0.21D
Mean differences in meridians of anterior and posterior corneal astigmatism
There were large range of differences in meridians of ant and post corneal astigmatism
Sample cases showing for the effect of posterior corneal astigmatism on total corneal astigmatism
Anterior astigmatism0.59D@87
Posterior astigmatism-0.35@90
corneal astigmatism from total cornea: 0.24@83
corneal astigmatism estimated fromanterior cornea: 0.53@87
If the meridians of ant and post corneal astigmatism are the same,posterior corneal astigmatism reduces anterior corneal astigmatism
Anterior astigmatism0.20D@13
Posterior astigmatism-0.26@103
corneal astigmatism from total cornea: 0.46@13
corneal astigmatism estimated fromanterior cornea: 0.18@13
If the meridian are 90-degrees apart, posterior corneal astigmatism increases anterior corneal astigmatism
Relationship between anterior and posterior astigmatism from the GalileiRatio in curvatures of posterior/anterior corneal astigmatism
Median range 25th to 75th percentile
Ratio in curvatures of posterior/anterior corneal
astigmatism0.34 0.05 to 1.07 0.14 to 0.43
There was wide variation for the ratio of post/ant corneal astigmatism
Conclusion Astigmatism measurements from all techniques were
highly reproducible and correlated
There were no significant differences in mean corneal cylinder between techniques
There was wide range of differences in steep meridian between anterior and posterior corneal astigmatism
Mean vector difference between anterior and total corneal astigmatism was 0.21D
Conclusion Our result indicates that posterior corneal
astigmatism compensates for astigmatism from anterior surface in some cases and augments it in others
Ignoring posterior corneal surface might yield incorrect estimation of total corneal astigmatism